Technical Field.
The present invention relates to a fuel delivery system according to the preamble of claim 1.
Background Information.
In the fuel delivery system of a commercial vehicle it is known to use a rotary displacement pump driven by the transmission of the vehicle to increase the fuel pressure in the system to a level suitable for injection of the fuel into the vehicle engine. The pump has to be capable of delivering fuel at a sufficient pressure substantially immediately upon starting the engine. This implies that at high engine speeds the pressure in the fuel delivery system is greater than actually required. Consequently, a pressure relief valve is required downstream of the pump to relieve the excess pressure. Should the pressure relief valve stick in a partially or fully closed position, there is a risk that the pressure in the fuel delivery system will become dangerously high, possibly resulting in rupture of a seal or fuel line.
A conventional rotary displacement pump comprises a housing, a pumping chamber within the housing, a driver rotor and a driven rotor within the pumping chamber, and an input shaft to the housing. The input shaft is connected to the driver rotor to effect rotation of the driver rotor. To prevent leakage of the pumped liquid from the pumping chamber, it is necessary that an adequate sealing means be provided between the housing and the input shaft. Due to the rotation of the input shaft, a dynamic seal must be employed. In the fuel delivery system described above, failure of the sealing means not only implies that fuel leaks out of the system, but also that the leaking fuel may migrate into the transmission and mix with the lubricant therein.
A fuel pump disclosed in U.S. Pat. No. 2,779,513 is driven by a power source via a magnetic coupling. A permanent impervious closure seals the pump from the power source, thereby reducing the risk of leakage. A spring pressed relief valve is provided downstream of the pump whereby fuel from the pump not consumed by a device, such as an internal combustion engine, is permitted to return back to the fuel tank.
It is an object of the present invention to provide a fuel delivery system suitable for use in a vehicle, with the system being more energy-efficient than conventional systems while less reliant on the necessity of a functioning pressure relief valve.
This object is achieved in accordance with the present invention by a fuel delivery system comprising a fuel reservoir connected to a suction side of a pump, a fuel delivery line connected to an output side of the pump, a number of fuel injectors connected to the delivery line, and a return line from the injectors to the suction side of the pump. The pump comprises a housing, a pumping chamber within the housing, a driver rotor and a driven rotor within the pumping chamber, and an input shaft to the housing, the input shaft being arranged such that rotation of said input shaft effects rotation of the driver rotor. The driver rotor is caused to rotate by the input shaft via a magnetic coupling. The magnetic coupling is arranged to slip when a predetermined value of torque is applied across the coupling such that a preferred maximum pressure value of about 12 bar is attained at the output side of said pump.
Since the magnetic coupling is only capable of transmitting a predetermined value of torque, the pressure downstream of the pump cannot exceed a predetermined value, irrespective of the rotational speed and/or torque of the input shaft.
In a preferred embodiment of the invention, the system further comprises a pressure relief valve in the fuel delivery line, the pressure relief valve being arranged to reduce the pressure in the fuel delivery line to about 6 bar.
Further preferred embodiments of the invention are detailed in the dependent claims.